1. Field of the Invention
The invention is related to the field of stirring apparatuses, and in particular, to a magnetically-coupled stirring apparatus and method.
2. Statement of the Problem
In laboratory or field testing situations, a sample fluid can be tested and analyzed for certain properties. Prior to such a test, it is generally desirable that the sample fluid be thoroughly mixed or stirred. This may be necessary because if the sample fluid has not been collected recently, then the contents may have separated, settled, or otherwise become non-uniform. An optimum test will rely on uniformity of the sample fluid. In addition, where the component or characteristic to be tested for is not plentiful, stirring of the sample fluid may bring a greater volume of the sample fluid into contact with a sensor.
In water quality testing, a water sample can be tested for oxygen content, such as the amount of oxygen that is dissolved in the water, for example. Such testing requires that the dissolved oxygen be uniformly dispersed in the water in order to obtain an accurate reading. Trending measurements of the dissolved oxygen can indicate the level of bacterial activity and/or a type of bacterial activity, for example. For wastewater and drinking water treatment, a measurement of dissolved oxygen may indicate whether the treatment process is being properly performed.
In natural waters, dissolved oxygen exists in a dynamic equilibrium that is controlled by biochemical depletion and oxygenation through atmospheric diffusion, aeration, and photosynthesis. However, as microbial growth in the water degrades organic matter, oxygen is consumed. Oxygen is re-supplied through atmospheric diffusion, aeration, and photosynthesis. As a result, bacterial populations proliferate and provide a key input on up the food chain.
The equilibrium of dissolved oxygen is subject to detrimental fluxes when a catastrophic event occurs, such as organic waste being discharged into the water. An immediate depletion in dissolved oxygen results in an anoxic environment. Depending on the severity of the event, the dissolved oxygen may be depleted to the point where higher trophic organisms such as macro invertebrates and fish are killed off.
In wastewater, organic-based sewage is degraded under controlled aerobic conditions. Failure to maintain adequate supplies of dissolved oxygen can result in anaerobic conditions that lead to offensive and corrosive sulfides. On the other hand, excessive aeration is wasteful and drives up operational costs.
When dissolved oxygen is reported in aeration basins and outfalls or used to derive the biochemical oxygen demand from wastewater, it becomes a regulatory tool. Thus, prudent monitoring of dissolved oxygen is essential for assessing environmental risk in natural waters. It is also necessary for optimal wastewater treatment performance and for ensuring regulatory compliance.
The level of dissolved oxygen can be measured using a Luminescent Dissolved Oxygen (LDO) sensor. The speed of the measurement performed by the LDO sensor is directly affected by the amount of stirring of the sample. The stirring affects the speed of the test by bringing the dissolved oxygen in the water into contact with or into the region of the emitted light and therefore accelerates the test process.
FIG. 1 shows a magnetic stirrer device of the prior art. The prior art magnetic stirrer device includes a stirring magnet (the upper magnet in the drawing) that rests on the bottom of a container. The prior art magnetic stirrer device includes a magnet that is rotated by a motor or other rotary power source, such as the lower magnet in the drawing. Rotation of the lower magnet about its axis causes the upper magnet to rotate about the same axis and follow the lower magnet. Consequently, the two magnets rotate about a common axis, i.e., they are coaxial.